You have evidently read a good deal and far more than most people of your age. The advice that I should give you is to master Monro's Homeric Grammar and Roby's large Latin Syntax (vol. ii). It is a great thing to be sure of your Homer and with that I should join a careful study of Sophocles and Thucydides  say 3 plays and Bks I-iii. Isocrates is in some ways the most important person in the 4th. century Gk. Lit. A mastery of Sandys' Panegyricus and of the Areopagiticus you will find really helpful. Translate also some of the de pace of Isocrates against some of the Olynthiacs. The foundation of all Greek scholarship is the knowledge of Homer and you can hardly do too much work at it. If you can, read Lehrs' Aristarchus and Wolf's Prolegomena through. In Latin I attach great importance to a knowledge of Plautus and Cicero’s letters to Atticus. Lucretius will I hope be one of your books for Moderations. Don't exhaust him now. Plautus can be read best with brief annotations and I advise you to get hold of Lindsay's notes on his grammar and syntax.

It is very important to have some subject of your own to work on hereafter. Thus Greek or Roman inscriptions. I think you would find the former very useful. Cauer or Dittenberger's Sylloge, or the little Teubner vol. of extracts. Or in Roman literature a study of the relation of Virgil and Horace to the great masters of prose style. For my own part I think Wilkins' de Oratore or Sandys’ Orator are simply admirable preparations for the Hertford Scholarship.

In composn. I recommend nothing but (1) translation from different styles side by side  e.g. Livy's III book and Cicero's Catilines, which from time to time put back; (2) a good deal of repetition; (3) try and visualize and make practical all you translate; (4) never show up a copy without translating all through, taking the words as if you were 12 years old.

P.S. Write to me in June and ask me to send you the subject for the Gaisford Greek Prizes and Latin Essay.